"Currently in mainline the balancing of multiple RT threads is quite broken. That is to say that a high priority thread that is scheduled on a CPU with a higher priority thread, may need to unnecessarily wait while it can easily run on another CPU that's running a lower priority thread," began Steven Rostedt, describing his patchset to introduce improved real time task balancing. He explained:
"Balancing (or migrating) tasks in general is an art. Lots of considerations must be taken into account. Cache lines, NUMA and more. This is true with general processes which expect high through put and migration can be done in batch. But when it comes to RT tasks, we really need to put them off to a CPU that they can run on as soon as possible. Even if it means a bit of cache line flushing. Right now an RT task can wait several milliseconds before it gets scheduled to run. And perhaps even longer. The migration thread is not fast enough to take care of RT tasks."
Steven described his test cases and numerous issues he noticed with the current balancing code, noting, "to solve this issue, I've changed the RT task balancing from a passive method (migration thread) to an active method. This new method is to actively push or pull RT tasks when they are woken up or scheduled."
Andrew Doran posted some threading benchmark results to NetBSD's tech-kern mailing list, following up to some benchmarks he'd posted earlier. The results compared NetBSD -current with FreeBSD -current, and the Linux 2.6.21 kernel. Kris Kennaway was surprised by the results, and ran his own benchmarks with minimal configuration changes, summarizing, "this measurement shows that FreeBSD is performing 70-80% better than NetBSD in this 4 CPU configuration. This is in contrast to Andrew's findings which seem to show NetBSD performing 10% better than FreeBSD on a 4 CPU system (a very old one though)." He added, "the drop-off above 8 threads on FreeBSD is due to non-scalability of mysql itself. i.e. it comes from pthread mutex contention in userland."
Kris ran additional benchmarks with PostgreSQL instead of MySQL, showing much improved scalability above 8 threads, "postgresql is much more scalable than mysql on this workload and doesn't have silly scaling bottlenecks inside the application (cf the tail of the FreeBSD curve for mysql which is where pthread mutex contention kicked in)." He continued his testing, and found that on older 4CPU P3 hardware NetBSD did outperform FreeBSD, "but only by 3-4% (in particular I am not seeing the ~10% difference that Andrew observes on his 4*p3 700MHz). Given the age of the hardware and the fact that I am not seeing it on other workloads or on modern hardware it might just be due to a small scheduling difference on this configuration."